chore: use ubuntu-20.04 instead of latest as the latest image will somtimes send a shutdown signal whilst a job is running (actions/runner-images#6709)

This seems to happen most often during our `publish` step (https://github.com/fastly/js-compute-runtime/actions/runs/4814102562)

Author: Jake Champion

.github/workflows/release-please.yml

@@ -63,7 +63,7 @@ jobs:
           git push --force
 
   publish:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     needs: release
     if: ${{ needs.release.outputs.releases_created }}
     steps: